1. Field of the Invention
This invention is related to the field of voltage controlled oscillators (VCOs) and particularly their use in phase locked loops (PLLs).
2. Description of the Related Art
PLLs are used in a wide variety of applications. In integrated circuits, especially modern integrated circuits operating at high clock frequencies, PLLs are often used to generate the internal clocks for the integrated circuit and to lock the internal clocks to the phase of the clock supplied externally. Phase locking the internal clock to the external clock may aid in providing reasonable setup and hold times for communications on the pins of the integrated circuit. PLLs are also used for clock recovery, data transmission/reception, etc.
Integrated circuits are often designed to be scaleable to different clock frequencies, allowing the integrated circuits to be marketed at different frequency levels/price points and enhancing the usefulness of the integrated circuit in a variety of systems. Accordingly, it is desirable for PLLs to operate properly over a wide range of frequencies while achieving a high degree of noise rejection and fast lock times (the amount of time needed to lock the phase of the internal clock to the external clock).
One factor affecting the frequency of operation of the PLL is a voltage controlled oscillator (VCO) within the PLL. Generally, the PLL measures the difference in phase and/or frequency between the internal clock and the external clock and controls the VCO to change the frequency of the internal clock until phase/frequency lock is achieved. Generally, it is desirable for the VCO to operate over a wide control voltage range (allowing the largest variation in the output frequency of the VCO) while still offering high noise rejection (to reduce errors induced by the noise). Additionally, the VCO must interface properly to the rest of the system, which may be operating over a different voltage range than the VCO.
A VCO for a PLL may include a ring oscillator having a power supply controlled in response to the VCO""s control voltage input and an inverter having an input coupled to the ring oscillator""s output and also supplied with a power supply controlled by the control voltage input. Together, the output of the ring oscillator and the output of the inverter may closely approximate a differential signal. The VCO may include an amplifier for amplifying a differential input to an output in the voltage domain of the system including the PLL. The output of the ring oscillator may be used as an input to the amplifier, and the output of the inverter may be used as the other input. Thus, an interface between the low voltages in the VCO and the system may be formed.
The power supply terminals of the ring oscillator and the inverter may be coupled to outputs of a current mirror. In one implementation, the current mirror may not be cascoded. Accordingly, the voltage range available for the ring oscillator (and thus the range of output frequencies available) may be larger than if a cascoded current mirror is used. The VCO may, in one embodiment, be supplied with a regulated voltage supply to improve the noise rejection of the VCO.
Broadly speaking, a circuit for use with a ring oscillator is contemplated. The circuit comprises a current mirror and an inverter. The current mirror has a first input for receiving an input current, a first output for providing a first output current proportional to the input current, and a second output for providing a second output current proportional to the input current. The first output is coupled to a first power supply terminal of the ring oscillator. The inverter is coupled to receive an output of the ring oscillator as an input and having a second power supply terminal coupled to the second output of the current mirror.
Additionally, a VCO is contemplated, comprising: a current mirror, a ring oscillator, and an inverter. The current mirror has a first input for receiving an input current, a first output for providing a first output current proportional to the input current and a second output for providing a second output current proportional to the input current. The input current is responsive to a control voltage input to the VCO. The ring oscillator has an output and a first power supply terminal. The first power supply terminal is coupled to the first output of the current mirror. The inverter has an input coupled to the output of the ring oscillator and has a second power supply terminal coupled to the second output of the current mirror.
Moreover, a method is contemplated. A voltage is provided on a power supply terminal of a ring oscillator responsive to a control voltage. The voltage is also provided on a power supply terminal of an inverter coupled to an output of the ring oscillator, whereby a signal swing on an output of the inverter is similar to a signal swing on the output of the ring oscillator.